What is Supplemental Insurance? Supplemental insurance is additional coverage that you can use to help with the out-of-pocket expenses that may not be covered by your major medical insurance. Have you ever had expenses not covered by your major medical plan? That’s where Aflac comes in. Aflac is voluntary insurance, or an optional layer of financial protection. And with healthcare costs on the rise, more Americans than ever are seeking an extra financial safety net for unexpected medical expenses.